This invention relates to a process wherein distillation residue produced in the phosgenation of a polyamine is recovered in reusable form. More particularly this invention relates to a process whereby said distillation residue is reconverted to the original polyamine in high yield.
In the phosgenation of polyamines to form polyisocyanates, the product polyisocyanate is generally distilled from the reaction mixture in which it is prepared. At the conclusion of the distillation, the reaction mixture normally contains a quantity of high boiling residue. Such residue generally comprises polymeric materials such as biurets, polycarbodiimides, diisocyanatocarbodiimides, polyuretdiones, isocyanurates and the like. Since this residue is not commercially useful it must be recycled or disposed of.
In theory, all such residue should be hydrolyzable to regenerate polyamines. In practice, however, complete hydrolysis has not been achieved due to the formation of insoluble lumps or masses which at best hydrolyze too slowly for commercially application.
It is known to use aqueous alkali metal hydroxide solutions to promote the hydrolysis of the distillation residue. However, such solutions fail to provide a decomposition product of uniform composition. In addition, such base-catalyzed hydrolysis reaction has not proceded in good yield nor has it solved the problem of insoluble lump formation.
In West German Pat. No. 2,703,313 it is taught to hydrolyze solid undistillable residue with aqueous ammonia or a primary or secondary amine solution. The primary or secondary amine may be the polyamine used in the preparation of the polyisocyanate, but the use of such polyamine is clearly stated not to be preferred.
Although the process of German Pat. No. 2703313 largely overcomes the problem lump formation and provides better yields than previous processes, several disadvantages remain. That process requires the addition of reagents or solvents in effecting the hydrolysis, which reagents or solvents must later be removed. Even though the polyamine used in preparing the polyisocyanate is said to be useful in the hydrolysis reaction, other materials such as ammonia, organic solvents, and .epsilon.-caprolactan are normally used in connection therewith. In addition, the yield of this process to polyamine is only about 50-70%, which is much lower than desirable.
It would be desirable to provide a process for the hydrolysis of distillation residue from the phosgenation of the polyamine, whereby a high yield of regeneration of the polyamine is achieved without the formation of substantial amounts of by-products.